As a conventional driving state estimation device, a technology described in JP 2009-009495 is known.
In the technology described in JP 2009-009495, a difference (hereinafter, also referred to as “distribution data”) between an estimated steering angle value under assumption that a steering operation is smoothly performed and an actual steering angle is calculated based on the steering angle of a steering wheel. Next, based on the calculated distribution data, a frequency distribution (hereinafter, also referred to as “traveling state distribution”) of the distribution data in a relatively long time range corresponding to an ordinary steering characteristic and a frequency distribution (the traveling state distribution) of the distribution data in a relatively short time range corresponding to a current steering characteristic are calculated.
Then, a driving state of a driver is calculated based on magnitude of a difference between the calculated two traveling state distributions. In this way, the technology described in JP 2009-009495 makes it possible to detect an unstable state of the driving with high accuracy regardless of a difference in a traffic environment. Here, in the technology described in JP 2009-009495, bins of the frequency distributions (the traveling state distribution), that is, data ranges segmented for classifying the distribution data are set in each of a positive value range and a negative value range.